The present invention relates to a method of glueing together two disc elements, especially substrates for digital audio, video or computer discs, such as digital video discs (DVDs).
When manufacturing certain types of data storage discs, such as so-called DVDs, with one or more information layers, two substrate discs of polycarbonate of a thickness of 0.6 mm are normally glued together, with the information layer(s) adjacent to the adhesive layer joining the discs.
In one proposed method of glueing, the adhesive is applied to one of the discs by means of a printing process, for example screen printing, whereafter the discs are joined together. This method does not waste adhesive and the application can be done very rapidly. However, there is some risk of air enclosures when the discs are put together and some risk that the printing system will break down should the screen printing blanket rupture.
A method using double adhesive tape for joining the two disc elements has also been suggested, but this method causes air enclosures and provides a joint of unreliable strength and thermal stability. Nor can this method be used for DVDs of the type xe2x80x9cSD9xe2x80x9d, i.e. where both of the discs elements have information layers and where these layers are to be read by laser light which must pass, with the correct defraction, through the joint layer between the disc elements.
One purpose of the present invention is to suggest a glueing method which prevents the occurrence of air enclosures in the adhesive layer between the discs.
For this purpose, the method according to the invention is characterized by the steps of:
a) bringing the two disc elements together coaxially with each other in such a manner that a gap is left between the disc elements;
b) applying, in an inner area of the gap, a liquid adhesive in such manner that the adhesive will essentially come into contact simultaneously with facing sides of the two disc elements, and
c) bringing the disc elements towards each other to achieve a well defined uniform filling out of adhesive between the disc elements.
It has been found that a simultaneous or essentially simultaneous wetting of the opposing gap surfaces with an easily flowing adhesive with good capillary filling properties is an essential precondition in order for the applied adhesive to flow out and be evenly distributed over the entire surface between the outer and inner peripheral limiting edges of the disc elements without forming air enclosures, since the disc elements are brought towards each other with only the aid of gravity and xe2x80x9care sucked togetherxe2x80x9d by the capillary forces of the adhesive. In this way a very uniform, homogeneous adhesive layer without air enclosures can be obtained without forceably pressing the discs together.
The disc elements can be kept plane-parallel to each other during the application of adhesive, or they can be held relative to each other in such a way during the application of adhesive that a radially outwardly diverging gap is formed between at least one radially outer section of the disc elements.
The diverging gap can be obtained by curving one or both of the disc elements convexly from each other, preferably by creating a subatmospheric pressure on the side of the disc element facing away from the gap.
The adhesive is applied preferably by injection via a nozzle insertable into the gap during simultaneous rotation of the disc elements, whereupon a coherent annular adhesive string can be laid out in the gap between the disc elements.
After application of the adhesive string, the nozzle is retracted and the two disc elements are rotated individually in opposite directions, each at least half a rotation, preferably about one half to one rotation, so that any uneveness in the circumferential distribution of the adhesive string can be evened out by the disc elements producing a shearing effect in the adhesive. The result will be a homogeneous and uniform ring of adhesive, which is an essential precondition for even distribution over the entire disc surface of the adhesive in a subsequent step.
The present invention also relates to a device for glueing together disc elements of the above mentioned type, said device comprising a lower holder for carrying one disc element, an upper holder for carrying the other disc element in a coaxial relation to the lower disc element to form a gap between the disc elements, and a means for applying a liquid adhesive in an inner area of the gap.
In order to create a diverging gap to facilitate insertion of the adhesive application means between the disc elements, there is one holder that can have an essentially convex surface facing the disc element. This convex surface is intended to define, together with the disc element and sealing elements a space which can be subjected to vacuum. Alternatively, the holders can be disposed to hold the disc elements plane-parallel.
The means for applying adhesive can comprise a nozzle which is disposed to direct, either radially from outside or inside, a stream of adhesive into the gap in such a way that the stream of adhesive strikes facing surfaces of the disc elements essentially simultaneously. The nozzle can be insertable into and extractable from the gap so that its opening comes close to the facing gap surfaces in an inner portion of the gap.
In order to provide a continuous annular string of adhesive in the gap, the holders are rotatable.
For the case when only one of the holders is arranged to curve an associated disc element to create the adhesive introduction gap, it is suitable to make the lower holder with a planar disc-carrying surface.